In recent years, the light bulb power or system thermal density in the present projection devices keeps rising due to the increasing demand for the high-brightness projection devices. In a projection device, not only the light bulbs, electronic components and power supply generate heat and thereby resulting in a high temperature while the projection device is being in use, the optical elements may also result in a high temperature while being emitted by light beams. To reduce the temperature, most of the existing projection devices are equipped with one or more axial fans or blowers for cooling the components in the projection device. However, in a high-dust environment, the airflow caused by the axial fan or blower may introduce the external dust into the inside of the projection device; and thus, the components in the projection device may have dust thereon.
Therefore, some projection devices, such as the ones disclosed in U.S. Pat. No. 7,161,315, U.S. patent publication No. 2007/0245701, U.S. patent publication No. 2007/0085976 and U.S. patent publication No. 2010/0077926 and Taiwan patents publication No. 200717163 and Taiwan patent No. 1247190, are equipped with a filter so as to prevent the dust from adhering to the internal elements of the projection devices.
However, after the filter used for a period of time, dust may block the meshes of the filter and consequently reduce the flow rate of airflow into the projection device, and thereby resulting in the projection device having a poor cooling efficiency.